Precious metals, and especially the platinum group metals (“PGM”), have been and continue to be used in the glass industry in processes for melting and heat-shaping special glass.
It is well known that PGM materials have high melting points, high heat stability, high mechanical strength, and resistance to abrasion. Because of these properties, PGM materials are particularly suitable for the production of construction parts that come into contact with high temperature liquids, such as glass melt. PGM materials suitable for parts that come in contact with glass melt include platinum, and alloys of platinum and/or other PGM metals, and may include minor amounts of base metals as alloys or additives. PGM materials typically used in the glass industry include fine platinum, PtRh10 (platinum-rhodium alloy with 10% rhodium,) or platinum that contains a small amount of finely divided refractory metal oxide, for example, zirconium dioxide. The use of zirconium dioxide with platinum increases the strength and high-temperature creep resistance of the platinum, and is called fine grain stabilized platinum (“FGS”).
Components and devices for molten glass products are typically used for melting, refining, transporting, homogenizing and portioning of molten glass, and are well known to persons skilled in the art.
These components for delivery of glass melt are usually constructions of PGM material. Alternatively, they may be made of other high heat stability materials such as for example, fireproof ceramics, or base metal materials with a thin-walled protective PGM covering. The covering may be in the form of thin sheet metal or a surface coating applied by methods known in the art, including but not limited to plasma spraying and flame spraying.
The components that carry glass melt are often precious metal sheets constructed as thin-walled pipe systems. The molten glass flows through the pipe systems with temperatures of between about 1000° C. and about 1700° C. The pipe systems are typically surrounded externally by an insulating and supporting ceramic held by supporting metal constructions, for example metal boxes.
A feeder device is typically used to feed in, or to meter and to portion the glass melt into the appropriate moulds of processing machines, for example, pressing machines, blowing machines, and press-blowing machines. The feeder is made up of a feeder head, feeder needle and feeder nozzle, also called a delivery nozzle. The use of feeders for glass melt delivery is well known to persons skilled in the art.
The temperature control in these components plays an essential role due to the high temperatures of operation, which is typically in the range of about 1000° C. to about 1700° C. To enable controlled supply or removal of heat, the components are heated electrically in some cases. This can be done either indirectly by additionally installed heating conductors, or directly by resistance heating of the PGM components of the nozzle. In the case of direct heating, the precious metal sheet acts as the heating conductor. The electrical energy introduced to the PGM sheet is converted into heat due to specific electrical resistance of PGM.
To be able to draw electrical energy into the component at the various places through the insulation, heating flanges or heating vanes are used. These establish the connection from the current source, for example the power cable or conductor rail of a transformer, to the precious metal component. The shaping and dimensions of the noble metal component and the shaping, dimensions and positioning of the current feeds are critical for carrying electrical current and for local evolution of heat during direct heating.
For portioned generation of glass drops or glass strands by the glass melt feeder, certain processing parameters must be adhered to precisely. In most cases, an accuracy of about ±1° C. processing temperature is necessary. It is essential for the weight of the glass drops or glass strands to be highly constant, in practice within a variation of about ±1%. In some cases, for example in the case of relatively small drops of less than 100 grams, the permitted deviation in weight is significantly less than one gram. Further, it may also be important to adhere to a defined drop shape, depending on the glass end product to be produced. In these cases, the length-width ratio of the drop requires particular attention.
The construction and temperature control of the delivery nozzle are critical to controlling glass temperature, drop weight and drop shape during the delivery of glass melt.
FIG. 1 shows in three detailed diagrams (a), (b) and (c) the construction of a feeder as typically known in the art, shown in axial half-section.
Detailed diagram (a) illustrates the fundamental construction of a typical feeder. The feeder comprises the conically shaped feeder head (1), in which the dome-shaped tip of the feeder needle (2) ends. The feeder nozzle, also called the delivery nozzle (3), comprises a conical funnel-shaped section (4) and a cylindrical end piece (5). The feeder nozzle sits flush on the feeder head (1). The feeder head and feeder nozzle are made of a thin PGM sheet and are surrounded on the outside by complementary and interlocking ceramic components (6), (7), against which they are held with PGM flanged rings (8), (9), (10) at the ends. The feeder nozzle has a take-off bar (11) that can protrude to a greater or lesser degree over the lower flanged ring (10). The delivery block (7) is mounted in a ring holder (12) of steel.
Such an unheated construction of the feeder device has the limitation that heat is withdrawn from the glass flux to an intolerable degree, especially in the critical region of portioning and shaping.
Detailed diagrams (b) and (c) show heated nozzle constructions according to the prior art.
Detailed diagram (b) shows an indirectly heated feeder nozzle in axial half-section (3′), which is fixed to the delivery block (7′) via flanged rings (9′), (10′) at the ends. The delivery block (7′) has spiral or twisting grooves on the inside (13), into which a heating coil (14) of noble metal, for example, platinum or platinum alloy wire is inserted. The connection to the electrical current supply (transformer) (15) passes through bores in the ceramic component (7′).
In this construction, the conical (4′) and in part the cylindrical (5′) section of the delivery nozzle (3′) and the delivery block (7′) are heated. However, the glass suffers the greatest heat loss on the under-side of the delivery nozzle due to radiation of heat and removal via the free surface of the flanged ring (10′) and take-off bar (11′). As a result, the lower part of the delivery nozzle cools to a greater degree at the take-off bar (11′), and therefore the glass cools at this particularly critical place.
Detailed diagram (c) shows a directly heated feeder nozzle in axial half-section, (3″), that is fixed to the interlocked delivery block (7″) via flanged rings (9″), (10″) at the ends. Current feeds (vanes) (16), (17) connected to the electrical current source (transformer) (15′) are shaped on to the flanged rings (9″), (10″). In this construction, the entire funnel of noble metal (3″) with the conical (4″) and cylindrical (5″) section is electrically directly heated.
This construction also has the limitation that more heat is withdrawn than can be supplied in the conical region, especially in the lower region, via the large free surface of the flanged rings.
The present invention is an improvement over these known constructions. According to present invention, a device comprised of an electrically directly heated delivery nozzle for glass melt that utilizes PGM material is provided. This device is constructed so that it does not have the disadvantage of excess heat withdrawal from the system. Additionally the device of the present invention provides regulatable, effective heating of the delivery nozzle and therefore of the glass flux, particularly in the critical region of portioning and shaping.